Temperature Dependence of Vibronic Transitions in Crystals and Spectroscopic Observation of a Phase Transition

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Abstract
The temperature dependence of vibronic absorption bands, corresponding to either one phonon absorbed or emitted simultaneously with an electronic transition in a crystal having an octahedral UCl62− complex was measured and found to agree with the theoretically expected results. A phase transition was observed by the weak appearance of the pure electronic transition above a transition temperature. The latter exhibits hysteresis favoring a first-order transition. Selection rules and general symmetry arguments show that the distortion must be one of D2, D3, S4, D2d (special) about the uranium site. The particular arrangement of atoms enables a further reduction in the possible distortions to D2d or D3. From the relative intensity of pure electronic to vibronic transitions one can estimate the size of the distortion to be of the order of 2×10−3 times the lattice constant.